The present invention relates to a throttling control valve for use in an oil pressure control circuit or the like and also to a system for controlling downshift timing of an automatic transmission for vehicles which employs this throttling control valve.
An automatic transmission for vehicles which comprises a fluid torque converter and a change-over gear mechanism including a plurality of friction engaging means for setting the transmission to various speed stages is so adapted that it is automatically set to the most desirable shift condition in accordance with the operating condition of the vehicle by automatically changing over engagement of the friction engaging means. Such an automatic change-over of engagement of the friction engaging means is generally controlled by an oil pressure control means which comprises a source of line pressure, a source of throttle pressure which increases in accordance with increase of the depression of the accelerator pedal or the opening of the intake throttle valve, a source of governor pressure which increases in accordance with increase of the vehicle speed, and a speed shift valve which is shifted between first and second positions so as to set the transmission to either of two speed stages in accordance with the balance between the throttle pressure and the governor pressure. When the speed shift valve is shifted to said first position, it connects the source of line pressure to a first friction engaging means which sets the transmission to a lower speed stage when it is engaged, while it connects a second friction engaging means, which sets the transmission to a higher speed stage when it is engaged, to a drain passage. On the other hand, when the speed shift valve is shifted to said second position, it connects the source of line pressure to the second friction engaging means, while it connects the first friction engaging means to a drain passage. In this connection, in order to provide a proper overlapping between the supply and the exhaust of oil pressure to and from the lower speed friction engaging means and the higher speed friction engaging means so as to accomplish smooth changeover between the two speed stages, the oil pressure control means generally further comprises accumulators connected individually to passages for supplying and exhausting oil pressure to and from the lower and higher speed friction engaging means.
In an automatic transmission of this kind, when the transmission is shifted down from the higher speed stage to the lower speed stage, the speed shift valve is shifted from the said second shift position to the said first shift position, and, in accordance with this, the line pressure which has been supplied to the higher speed friction engaging means by way of the speed shift valve is now exhausted toward a drain passage, while on the other hand the lower speed friction engaging means which has been connected to a drain passage is now supplied with the line pressure by way of the spaced shift valve. In this case, in accordance with the shifting of the speed shift valve, the oil pressure in the higher speed friction engaging means lowers immediately and rapidly by a certain amount, and thereafter this pressure level is maintained for a while at an accumulator pressure level because the exhaust of oil is compensated for by the delivery of oil from the higher speed accumulator due to shifting of its piston. After the piston of the accumulator has shifted through its stroke, the oil pressure again rapidly lowers, and at a middle portion of the lowering process the higher speed friction engaging means is substantially disengaged. On the other hand, although the lower speed friction engaging means is supplied with oil pressure immediately after the change-over of the speed shift valve, the supply of oil in the initial stage is consumed for driving the piston of the lower speed friction engaging means through its clearance stroke, and therefore the pressure level in the lower speed friction engaging means does not make any substantial rise. After the piston has moved through its clearance stroke, the oil pressure in the lower speed friction engaging means begins rapidly to rise, and at a middle portion of its rising process the lower speed friction engaging means is substantially engaged. Thereafter the oil pressure is maintained for a while at a certain accumulator pressure level as the piston of the lower speed accumulator moves and absorbs the supply of oil, and, after the piston of the accumulator has moved through its stroke, the oil pressure in the lower speed friction engaging means rises finally to the level of the line pressure. In order to accomplish this downshifting due to the change-over of oil pressure smoothly without causing a shift shock to the vehicle, there must be a proper time lag between the time point at which the higher speed friction engaging means is substantially disengaged in the course of exhausting oil pressure from the higher speed friction engaging means and the time point at which the lower speed friction engaging means is substantially engaged in the course of supplying oil pressure to the lower speed friction engaging means, because for smooth shifting it is required that the rotational speed of the engine at the instant of engaging the lower speed friction engaging means should be higher than the rotational speed of the engine at the instant of disengaging the higher speed friction engaging means by an amount corresponding to the ratio between the reduction ratios of the lower and higher speed stages, and in order to accomplish this increase of rotational speed the engine requires a certain time after the disengagement of the higher speed friction engaging means.
In connection with this required increase of the rotational speed of the engine, it must be noted that although the rate of increase required for the rotational speed of the engine is constant, the absolute value of increase required for the rotational speed of the engine differs in accordance with the rotational speed of the engine before the downshifting, i.e. the vehicle speed before the downshifting. For example, let us assume that the proportional increase of the reduction ratio due to downshifting is 1.5. If the rotational speed of the engine before downshifting is 2000 rpm, it is required that the engine should increase its rotational speed by 1000 rpm, from 2000 rpm to 3000 rpm. On the other hand, if the rotational speed of the engine before downshifting is 4000 rpm, it is required that the engine should increase its rotational speed by 2000 rpm, from 4000 rpm to 6000 rpm. Therefore, when the vehicle speed is higher before downshifting, the increase of the rotational speed of the engine required for smooth downshifting is larger. Although the rate of increase of the rotational speed of the engine is also affected by the magnitude of the engine torque, i.e. the opening of the intake throttle valve or the level of the throttle pressure, the time required for accomplishing the increase of the rotational speed of the engine required for smooth downshifting, i.e. the time interval between the time point at which the higher speed friction engaging means is substantially disengaged and the time point at which the lower speed friction engaging means is substantially engaged, differs in accordance with the vehicle speed before downshifting, said time interval being required to be larger when the vehicle speed before downshifting is higher.
In view of the abovementioned requirement, it has been proposed in Japanese Patent Application No. 94523/71 (Japanese Patent Publication No. 18344/77), filed by the assignee company of the present application, to incorporate a throttling control valve in a passage for supplying oil pressure to the lower speed friction engaging means, said throttling control valve being adapted to be changed over by the governor pressure so as to provide different throttling ratios to the passage in accordance with the magnitude of the governor pressure. The throttling control valve proposed in the former application is an ON/OFF valve which controls a bypass passage which bypasses a throttling element provided in the passage for supplying oil pressure to the lower speed friction engaging means and operates so as to open the bypass passage when the governor pressure is lower than a predetermined level so that the supply of oil pressure to the lower speed friction engaging means is performed more quickly and so as to interrupt the bypass passage when the governor pressure is higher than the predetermined level so that the supply of oil pressure to the lower speed friction engaging means is performed more slowly. In this former proposal, the throttling ratio applied to the passage for supplying oil pressure to the lower speed friction engaging means is changed between two values in accordance with the magnitude of the governor pressure.